A headup display device disclosed in Patent Literature 1 is known as a conventional headup display. The headup display device comprises a display device, a collimator lens, and a pair of plane mirrors arranged in parallel. One of the pair of plane mirrors is a semi-transparent mirror that reflects a part of incident light and transmits a part thereof. Light (display light) transmitted from the display device enters the collimator lens and becomes parallel light, and enters a pair of plane mirrors arranged in parallel. As one of the plane mirrors arranged in parallel is a semi-transparent mirror, a part of the pair of parallel light entered into the semi-transparent mirror is emitted from the semi-transparent mirror. The parallel light entered into the pair of plane mirrors arranged in parallel repeats reflection between the plane mirrors. Thus, a plurality of light is emitted from the semi-transparent mirror. The plurality of parallel light emitted from the semi-transparent mirror is reflected by a plane transparent plate (so-called a combiner), and the parallel light reaches an eye of an observer. As the parallel light enters an eye of an observer, the observer recognizes as if a display image is present in a distant place by viewing a virtual image projected onto the combiner.
Sense of distance (perspective) of the display will be briefly described. As shown in FIG. 10, the observer recognizes the distance to an object G1 by an angle formed by a line of sight E1 to the object G1 incident on a right eye 3R and a left eye 3L. When an object is present in a distant position (G2), an angle formed by a line of sight E2 to an object G2 incident on the right eye 3R and the left eye 3L decreases, and the observer recognizes as if the object G2 is present in a distant place. Light emitted from the semi-transparent mirror described in Patent Literature 1 is, as shown in FIG. 11, enters substantially parallel to the right eye 3R and the left eye 3L of the observer, the angle formed by a line of sight E3 to the right eye 3R and the left eye 3L becomes zero (parallel). Thus, it is possible to recognize as if the object G3 is present in a distant place (infinite). Further, as a plurality of parallel light is emitted from the semi-transparent mirror described in Patent Literature 1, it is possible to ensure a wide viewable area where a virtual image can be viewed even when the positions of the right eye 3R and the left eye 3L are moved, as shown in FIG. 11.
In such a configuration, by being kept in parallel, a pair of plane mirrors repeating reflection stably emits a plurality of light from a semi-transparent mirror at a predetermined emission angle.